addChildren example

public function getNestedLineItems(): ?OrderLineItemCollection
    {
        $lineItems = $this->getLineItems();

        if (!$lineItems) {
            return null;
        }

        /** @var OrderLineItemCollection $roots */
        $roots = $lineItems->filterByProperty('parentId', null);
        $roots->sortByPosition();
        $this->addChildren($lineItems, $roots);

        return $roots;
    }

    public function getAffiliateCode(): ?string
    {
        return $this->affiliateCode;
    }

    public function setAffiliateCode(?string $affiliateCode): void
    {